Persistent pulmonary hypertension (PPHN) of the newborn is a relatively common disorder with significant morbidity and mortality. Infants that do not respond to conventional therapies must be emergently transported to tertiary care centers that provide advanced therapies (i.e. ECMO,nitric oxide). Magnesium sulfate, if found effective in augmenting oxygenation, may reduce the need for these costly and advanced therapies in an inexpensive and safe manner. The hypothesis is that intravenous magnesium sulfate (MgSO4) administered to newborn infants with PPHN will result in significant improvement in oxygenation and reduce the need for more aggressive therapies. Status: Study terminated due to concerns about possible adverse effects of MgSO4.